What are the sources of chemical contamination resulting from urbanisation?
In modern cities, roads cover up to 30% of the land. This provides a huge surface area for runoff of contaminants (e.g., oils, grease, and hydrocarbons) into waterways via stormwater drains that collect water from roads, car parks, and paved surfaces. Asphalt pavement contains binder material that includes petroleum with varying amounts of polycyclic aromatic hydrocarbons (PAHs). Old asphalt material is broken up and reused either as a sub-base or in new asphalt pavement. In the environment, PAHs can bind to suspended particles (i.e., sediment) and accumulate in aquatic organisms. The most common source of PAHs in waterways is from airborne PAHs (burning of wood and other carbon materials). Other sources include municipal waste, urban stormwater runoff, effluent from wood treatment plants, and oil and petrol spills.
Contaminants found in the soil in landfills can also leach into groundwater, and then into a waterway. Other potential water quality issues include releases of toxic substances from wastewater treatment plants and industrial discharges (e.g., ammonia, chlorine, heavy metals, organochlorine pesticides, mercury, and organic contaminants), many of which can bioaccumulate and biomagnify in aquatic species and have acute or long term toxic effects.
Potential impacts of chemical contaminants on water quality and mahinga kai
- Local loss of fish species - fish may be harmed by contaminated water. Discharges and runoff into rivers and streams can be lethal to aquatic life, depending on the strength of the contaminant and size of the waterway.
- Local loss of invertebrate species - contaminants can be lethal to invertebrates. Invertebrates are also food for fish and persistent discharges that kill invertebrates could cause fish to travel farther in search of food, exposing them to greater risks and stress.
- Decreased dissolved oxygen (DO) levels - waste compounds released into waterways initiate biochemical reactions that use up oxygen as the stream bacteria break down the organic matter (Biogeochemical Oxygen Demand, BOD). Excess nutrients can also lead to algal blooms, and oxygen is used up when the algae die and decompose. Fish ‘breathe’ oxygen through their gills; a decrease in available oxygen (anoxia) in the water column threatens their ability to respire, which may lead to death. Fish that tolerate low levels of dissolved oxygen (such as the introduced species gambusia) may replace native populations that are less tolerant.
- Increased turbidity and decreased water clarity - water may become cloudy or discoloured with chemical contamination, reducing the ability of fish to see prey and detect predators.
- Damage to species - repeated exposure to sub-lethal doses of some contaminants can cause physiological and behavioural changes in fish that have long term effects on the population, such as reduced reproductive success, abandonment of nests and broods, a decreased immunity to disease, tumours and lesions, impairment of the central nervous system, and increased failure to avoid predators.
- Some contaminants (such as mercury) may bioaccumulate in animal tissues and be carried to human consumers of the fish.
Learn more about the potential environmental impacts of chemical contaminants in waterways
